i was stuck on seymour at mount gagazet for ages, so what i did was this

got all my ppl overdrives

got all my aeons overdrives

whooped seymour with overdrives then got the aeons out

you get one move with the aeon before he banishes them so use the overdrive

use hastega if possible as it helped me.

have the characters with highest hp out for most of the battle incase he uses crosscleave

have yuna out for the aeons

sorry if this isnt helpful.